The present invention generally relates to a laser driver for an optical signal transmitter in an optical communication system like a passive optical network (PON) system. More particularly, the present invention relates to a laser driver with an automatic power control (APC) function of maintaining constant optical output power responsive to a burst signal and irrespective of a variation in temperature.
In recent years, the optical subscriber systems are under vigorous research and development to set up a fiber-to-the-home (FTTH) communication network in the near future. However, it is economically difficult to introduce optical fibers into general home users. This is because an optical fiber has a gigantic transmission capacity, but is much more expensive than a conventional metallic communication line. Under the circumstances such as these, the PON system is expected to contribute much to the realization of the FTTH network considering the cost effectiveness thereof. The PON system can be less expensive, because a signal optical fiber extended from a base station is branched to provide bidirectional communication service for a great number of subscribers.
It is known that a semiconductor laser diode, which is ordinarily used for transmitting an optical signal in such an optical communication system, is highly temperature-dependent owing to the characteristics thereof. Specifically, if the laser diode is driven with a constant current supplied, the optical output power thereof decreases considerably with the rise in temperature. In addition, the performance of the laser diode deteriorates with time, and the optical output power also declines after years of operation. Accordingly, in general, a laser driver is constructed in such a manner as to maintain constant optical output power by monitoring the optical output power of the laser diode using a photodiode (PD) and feeding the information back to a laser driving section.
An exemplary laser driver with such a construction is disclosed in Japanese Laid-Open Publication No. 5-63273, for example. In this prior art laser driver, the optical output power of a laser diode is received by a monitoring photodiode. The output current of the monitoring photodiode is converted into a voltage. A difference between this voltage and a reference voltage is amplified by a differential amplifier. And then a current-supplying transistor is driven with the output of the differential amplifier.
According to the conventional construction, however, if the amplification factor of the amplifier for amplifying the difference between the output voltage of the monitoring photodiode and the reference voltage has varied due to a change in temperature, for example, the output current driving the laser diode also changes. Thus, it is difficult to attain high-precision APC performance in such a situation.
An object of the present invention is providing a laser driver realizing high-precision APC performance irrespective of the variation in characteristics of the components thereof.
Specifically, a laser driver for driving a laser diode according to the present invention includes a current-to-voltage converter, a reference voltage generator, a transconductor, a holding capacitor and a drive current output circuit. The current-to-voltage converter converts an output current of a photodiode, which is provided to monitor the optical output of the laser diode, into a voltage and then outputs the voltage as a monitor voltage. The reference voltage generator generates a predetermined reference voltage. The transconductor receives the monitor and reference voltages and supplies an output current corresponding to a difference between these input voltages. The holding capacitor is connected to an output terminal of the transconductor. And the drive current output circuit receives a voltage held by the holding capacitor as an input voltage and outputs a laser drive current corresponding to the input voltage. The laser driver is constructed to perform a feedback control in such a manner as to equalize the monitor voltage with the reference voltage.
According to the present invention, a feedback control is performed in such a manner to equalize the monitor voltage with the predetermined reference voltage. Thus, even if the current-to-optical-output conversion efficiency of the laser diode has changed due to variation in temperature or deterioration with time, the optical output power of the laser diode is kept constant. As a result, high-precision and stabilized APC performance is realized.
In one embodiment of the present invention, the current-to-voltage converter may include: an inverting amplifier receiving the output current of the photodiode; and a feedback resistor connected between input and output terminals of the inverting amplifier. In such an embodiment, the current-to-voltage converter is implementable to have a simple configuration.
In an alternate embodiment, the current-to-voltage converter may include: a transistor, the source and drain of which are connected to the input and output of the current-to-voltage converter, respectively, a constant voltage being applied to the gate of the transistor; a constant-current power supply connected to the source of the transistor; and a resistor connected to the drain of the transistor. In such an embodiment, the current-to-voltage converter can perform the current-to-voltage conversion stably with reduced power dissipation.
In another alternate embodiment, the predetermined reference voltage generated by the reference voltage generator is preferably controllable responsive to an external input. In such an embodiment, the optical output power of the laser diode is easily adjustable by controlling the reference voltage generated by the reference voltage generator.
In still another embodiment, the transconductor may include: a differential input stage for receiving two input voltages; a first current mirror circuit, the input of the first current mirror circuit being connected to one of two outputs of the differential input stage; a second current mirror circuit with two outputs, the input of the second current mirror circuit being connected to the other output of the differential input stage; a third current mirror circuit with two outputs, the input of the third current mirror circuit being connected to the output of the first current mirror circuit, the two outputs of the third current mirror circuit being connected to the two outputs of the second current mirror circuit, respectively; a fourth current mirror circuit including transistors of one conductivity type, the input of the fourth current mirror circuit being connected to one of the two outputs of the third current mirror circuit; and a fifth current mirror circuit including transistors of the other conductivity type, the input of the fifth current mirror circuit being connected to the other output of the third current mirror circuit. The outputs of the fourth and fifth current mirror circuits are connected to each other at a node, from which the current is output. In such an embodiment, an output current corresponding to a difference between the two input voltages can be obtained easily.
In still another embodiment of the present invention, the laser driver may further include a control signal generator for controlling the operations of the drive current output circuit and the transconductor responsive to a data signal. The control signal generator preferably activates the transconductor after a prescribed time has passed since the drive current output circuit was made to start to output the laser drive current by the control signal generator. In such an embodiment, it is possible to eliminate an error, which is caused by a delay between the output of the laser drive current and the conversion of the output current of the photodiode into the monitor voltage.
In still another embodiment, the laser driver may further include an adaptive bias circuit for charging or discharging the holding capacitor in such a manner as to reduce a difference between the monitor and reference voltages if the difference is larger than a predetermined value. In such an embodiment, high-speed APC response is realized.
In this particular embodiment of the present invention, the laser driver may further include a control signal generator for controlling the operations of the drive current output circuit and the adaptive bias circuit responsive to a data signal. The control signal generator preferably activates the adaptive bias circuit if the difference between the monitor and reference voltages is larger than the predetermined value after a prescribed time has passed since the drive current output circuit was made to start to output the laser drive current by the control signal generator. In such an embodiment, it is possible to eliminate an error, which is caused by a delay between the output of the laser drive current and the conversion of the output current of the photodiode into the monitor voltage.
In yet another embodiment, the laser driver may further include a transconductance controller for controlling a transconductance of the transconductor based on the predetermined reference voltage. In such an embodiment, the fluctuation in APC response time, which results from the assembling-induced variation, can be suppressed.
In this particular embodiment, the transconductance controller may control the transconductance by changing a bias current of a differential input stage included in the transconductor. In such an embodiment, the transconductance of the transconductor can be controlled easily.
In an alternate embodiment, the transconductor may include first and second differential input stages with mutually different transconductances. And the transconductance controller may control the transconductance by changing a bias current ratio of the first and second differential input stages. In such an embodiment, the transconductance can be controlled in a broader range.
In still another embodiment, a voltage held by a holding capacitor is preferably limited such that the laser drive current does not exceed a predetermined upper limit. In such an embodiment, it is possible to prevent the laser diode from being broken down due to an excessively large drive current.
In still another embodiment, the laser driver may further include an alarm circuit for outputting an alarm signal if a voltage held by the holding capacitor exceeds a predetermined upper limit and if the monitor voltage is smaller than a predetermined lower limit. In such an embodiment, a fault of the laser diode, if any, can be spotted easily.
An optical transceiver according to the present invention is adapted to establish an optical communication. The optical transceiver includes: a transmitter section for converting data to be transmitted into laser light by driving a laser diode and then transmitting the laser light; and a receiver section for converting the laser light received into received data. The transmitter section includes the laser driver of the present invention and drives the laser diode using the laser driver.